Method of improving the luminous efficiency of a sequential-colour matrix display

ABSTRACT

The present invention relates to a method of improving the luminous efficiency of a sequential-colour matrix display, the display being driven using an addressing method of the pulse width modulation or PWM type. This method comprises, for each pixel of a subframe, the following steps: comparison of the pixel colour value of the preceding subframe with a reference value so as to provide an overlap value depending on the period of overlap with the current subframe; if the pixel colour value of the current subframe less the overlap value gives a positive value, a time offset is added to the pixel colour value of the current subframe; if the pixel colour value of the current subframe less the overlap value gives a negative value, the pixel colour value of the current subframe is forced to be zero. The invention applies to LCOS or LCD displays.

The present invention relates to a method of improving the luminousefficiency of a sequential-colour matrix display. It relates especiallyto matrix displays in which the electrooptic valve consists of aliquid-crystal valve, more particularly a valve of the LCOS (LiquidCrystal On Silicon) type.

Liquid-crystal display (LCD) panels used in direct viewing displays orin projection displays are based on a matrix scheme with an activeelement at each pixel. Various addressing methods are used to generatethe grey levels corresponding to the luminance to be displayed at theselected pixel. The most conventional method is an analogue methodwhereby the active element is switched for a line period in order totransfer the analogue value of the video signal to the capacitor of thepixel. In this case, the liquid crystal material is oriented in adirection that depends on the value of the voltage stored on thecapacitor of the pixel. The incoming light polarization is thenmodified, and analysed by a polarizer so as to create the grey levels.One of the problems with this method stems from the response time of theliquid crystal, which depends on the grey levels to be generated. Thus,when this method is used to drive the electrooptic valve of asequential-colour matrix display in which the electrooptic valve,especially the LCOS valve, is successively illuminated with red, greenand blue colour filters, the very short response time between theintermediate grey levels results in very poor saturation of the coloursin the image when one colour is not completely eliminated duringillumination by the next colour.

To remedy this type of drawback, there has been proposed in the priorart, for example in the patent U.S. Pat. No. 6,239,780, a method ofdriving a matrix display using a pulse width modulation or PWMtechnique. In this case, the pixels of the liquid-crystal display areaddressed in on/off mode, the “on” mode corresponding to saturation ofthe liquid crystal. The grey levels are given by the width of the pulse.With such an addressing method, the dynamics of the display panel areimproved since the transition time now represents only a smallproportion of the total opening time of the liquid-crystal cell,whatever the value of the luminance.

This addressing method is particularly beneficial when it is used with asequential-colour optical engine using a single electrooptic valve, moreparticularly a LCOS valve, which is illuminated in succession with thecolours red, green and blue. This method, since an on/off mode is used,benefits from a more rapid response time, this being constant whateverthe grey level that has to be rendered.

However, although this method has the advantage of improving theresponse time of the liquid crystal and thus of obtaining optimum coloursaturation for the video content, nevertheless the luminous efficiencydecreases proportionally with the response time of the liquid crystal.

The object of the present invention is therefore to provide a method forimproving this efficiency in the case of a sequential-colour matrixdisplay, in which the display is driven using an addressing method ofthe pulse width modulation or PWM type.

Consequently, the subject of the present invention is a method ofimproving the luminous efficiency of a sequential-colour matrix display,the display being driven using an addressing method of the pulse widthmodulation or PWM type, characterized, for each pixel of a subframe, bythe following steps:

-   -   comparison of the pixel colour value of the preceding subframe        with a reference value so as to provide an overlap value        depending on the period of overlap with the current subframe;    -   if the pixel colour value of the current subframe less the        overlap value gives a positive value, a time offset is to be        added to the pixel colour value of the current subframe;    -   if the pixel colour value of the current subframe less the        overlap value gives a negative value, the pixel colour value of        the current subframe is forced to be zero.

According to another feature of the present invention if the pixelcolour value of the current subframe less the overlap value gives anegative value, the pixel colour value of the preceding subframe and thecolour value of the next subframe are modified so as to maintain theoriginal tint, while at the same time reducing the luminance.

In accordance with the present invention, the steps described aboveapply in succession to each sequential colour of a frame. Moreover, thepixel colour value of a subframe depends on the width of the PWM-typeaddressing pulse. The reference value depends on the response time ofthe material forming the display and the time offset depends on theresponse time of the material forming the display and on the duration ofthe subframe.

Other features and advantages of the present invention will becomeapparent on reading the description given below of one embodiment of thepresent invention, this description being given with reference to thedrawings appended hereto, in which:

FIG. 1 is a schematic representation of a matrix display driven using anaddressing method of the pulse width modulation or PWM type, to whichthe present invention can apply;

FIGS. 2 a to 2 e show the various signals for driving the display ofFIG. 1;

FIGS. 3 a to 3 c are curves giving the luminance value in the case of adisplay driven using a PWM-type addressing method, whereby saturation ispreserved;

FIGS. 4 a to 4 c are figures similar to FIGS. 3 a to 3 c in the case inwhich priority is given to luminance as opposed to colour saturation;

FIGS. 5 a to 5 c are figures identical to FIGS. 3 a to 3 c and 4 a to 4c giving the luminance obtained in the case of the method of the presentinvention;

FIG. 6 is a diagram in block form of a circuit for implementing themethod of the present invention;

FIG. 7 is a diagram in block form showing the circuit of FIG. 6 appliedto the three colours red, blue and green;

FIG. 8 is a diagram giving the luminance as a function of time, allowingthe principle applied in the present invention to be explained; and

FIGS. 9 and 10 are luminance curves explaining the correction functionapplied in the present invention.

To simplify the description in the figures, the same or similar elementswill have the same references.

We will firstly describe, with reference to FIG. 1, an embodiment of amatrix display to which the present invention may apply. This matrixdisplay comprises an electrooptic valve, more particularly a LCOS-typedisplay panel. FIG. 1 shows very schematically a picture element orpixel 1 of the display panel. This pixel 1 is indicated symbolically bya capacitor Cpixel connected between the back electrode CE and, in theembodiment shown, the output of a voltage-time converter 2 forimplementing an addressing method of the pulse width modulation or PWMtype.

As shown schematically, the voltage-time converter 2 comprises anoperational amplifier 20 whose negative input receives a ramp-shapedsignal, labelled Ramp, and whose other input receives a positive voltagecorresponding to the charge on a capacitor 21. The charge on thecapacitor 21 is controlled by a switching system, more particularly atransistor 22 mounted between one electrode of the capacitor and theinput of the voltage-time converter. This switching device consists of atransistor whose gate receives a pulse, labelled Dxfer.

As shown in FIG. 1, the picture element or pixel 1 is connected to a rowN and a column M of the matrix via a switching circuit such as atransistor 3. More specifically, the gate of the transistor 3 isconnected to a row N of the matrix, which is itself connected to a rowdriver 4. Moreover, one of the electrodes of the transistor, for examplethe source, is connected to the input of the voltage-time converter 2,while the other electrode or drain is connected to one of the columns Mof the matrix, this column being connected to a column driver 5 whichreceives the video signal to be displayed. Moreover, a capacitor Cs ismounted in parallel with the pixel capacitor as input to thevoltage-time converter in order to store the video signal value when thesaid pixel is selected. The column driver 5 and row driver 4 areconventional circuits. The column driver 5 receives the video signal tobe displayed, “Video in”, and is controlled by a clock signal Cclk and astart pulse Hstart. The row driver 4 allows the rows to be addressedsequentially and receives a clock signal Rclk and a start pulse Vstart.

The mode of operation of the display panel when it is used in asequential-colour display, namely when, during a frame T, a wheelcarrying three, green, blue and red, colour filters makes one completerevolution in order to illuminate the valve sequentially, will beexplained with reference to FIGS. 2 a to 2 e.

As shown in FIG. 2 a, a pulse I is applied at the start of each subframeT/3 to the row N so as to turn on the switching transistor 3. When theswitching transistor 3 is turned on, the capacitor Cs charges up to avoltage corresponding to the video signal present on the column M. Thatis to say, if a green colour filter lies opposite the display during thefirst subframe T/3, the capacitor Cs charges up to a value labelledVgreen in FIG. 2 b. During the next subframe, namely at time T/3, a newpulse I is applied to the row N, allowing the capacitor Cs to charge upto a voltage labelled Vblue, corresponding to the colour blue lying atthat moment opposite the display. Likewise, at time 2T/3, a new pulse Iis applied to the row N and the capacitor Cs charges up to a voltagelabelled Vred in FIG. 2 b. With the display in FIG. 1 driven using a PWMaddressing method, the values Vgreen, Vblue, Vred stored in successionon the capacitor Cs are applied to the capacitor Cpixel via thevoltage-time converter 2 which operates in the following manner.

A pulse I′ is applied within a subframe to the gate Dxfer of theswitching transistor 22 so as to turn it on. In this case, the voltagestored on the capacitor Cs is transferred to the capacitor 21 mounted inparallel and connected to one of the input terminals of the operationalamplifier 20. As shown in FIG. 2 d, at the end of the pulse I′ appliedto the gate Dxfer, a ramp r is applied to the negative input of theoperational amplifier 20. In this way, a voltage Vpixel, the duration ofwhich corresponds to the voltage Vgreen stored on the capacitor 21, isobtained as output from the operational amplifier 20, as shown in FIGS.2 d and 2 e. The same applies in the case of the subframes thatcorrespond to the passing of the blue and red colour filters in the casein which the display in FIG. 1 is used for sequential colour display.

We will now explain, with reference to FIGS. 3 a to 3 c, 4 a to 4 c and5 a to 5 c, the problem that the method of the present invention seeksto solve, this being applied especially to a matrix display like thatdescribed with reference to FIG. 1.

FIGS. 3 a to 3 c show the luminance values obtained when it is desiredto have saturated colours. In this case, it may be clearly seen that theloss of luminous efficiency is due to the fact that the liquid crystalin the case of an LCOS valve requires long rise and fall times, namelyof a few milliseconds. Thus, in FIG. 3 a, which shows a 100% saturatedred pixel being addressed, the subframe labelled Red receives a 100%luminance signal R1 over the duration of the subframe, whereas thesubframes labelled Blue and Green receive no signal. There is no overlapbetween the colours and colour saturation is maintained. FIG. 3 b showsthe addressing of a pastel red pixel. In this case, the subframe Red isaddressed by a pulse R1 throughout the duration of the subframe, whereasthe subframes Blue and Green are addressed by pulses R2, R3 for ashorter time. In this case too, in order to maintain saturation of thecolours, there is no overlap of the colours of one subframe withanother. FIG. 3 c shows the addressing of a white pixel. In this case,each subframe, Red, Blue, Green, is addressed by identical pulses R1,R2, R3 over the entire period of each subframe. Because of the pulserise and fall times, a loss of luminous efficiency shown symbolically bythe bold lines between each pulse in FIG. 3 c, is observed. FIGS. 4 a, 4b and 4 c are figures identical to FIGS. 3 a, 3 b and 3 c, but in thecase in which priority is given to luminance and not to coloursaturation. In the case of a 100%-saturated red pixel being addressed,as shown in FIG. 4 a, the pulse R1 is therefore applied during the Redsubframe over a period t1 greater than the time T/3, so that the pulsefall time overlaps the subframe labelled Blue. In this way, some of theblue light passes through the red, producing a pink pixel. FIG. 4 bshows the case in which a pastel red pixel is being addressed. In thesame way, the Red subframe is addressed by a 100% saturated pulse R1,with a pulse fall time starting at the end of the subframe andoverlapping the Blue subframe. The Blue subframe is addressed by a 30%Blue pulse R2 and the Green subframe by a 30% Green pulse R3. Since theGreen pulse does not have the same starting point, a time offset t2 mustbe added in order to compensate for the rise time of the liquid crystal,as shown by the solid and dotted lines in FIG. 4 b.

FIG. 4 c shows a white pixel being addressed. In this case, a perfectwhite is obtained in the case of the Red, Blue and Green subframes, asshown by the single pulse R.

The results obtained with the method used in the present invention toimprove the luminous efficiency will now be described with reference toFIG. 5 a, 5 b and 5 c.

In this case, the method used consists, for each pixel of a subframe, incomparing the pixel colour value of the preceding subframe with areference value so as to deliver an overlap value that depends on theperiod of overlap with the current subframe and then, if the pixelcolour value of the current subframe less the overlap value gives apositive value, a time offset is to be added to the pixel colour valueof the current subframe, and if the pixel colour value of the currentsubframe less the overlap value gives a negative value, the pixel colourvalue of the current subframe is forced to be zero.

The results of this method are shown, for example, in FIG. 5 a in which,during the subframe labelled Red, a 100% luminance signal R1 is appliedand the dotted part R′ shows that colour saturation is maintained whenthe Red subframe is addressed, while slightly reducing the luminance byan amount equivalent to the overlap time represented by the hatchedpart.

According to a variant of the method, if the pixel colour value of thecurrent subframe less the overlap value gives a negative value, thepixel colour value of the preceding subframe and the colour value of thenext subframe are modified so as to maintain the original tint, while atthe same time reducing the luminance. This is shown, for example, inFIG. 5 b, which gives an example of a pastel red pixel being addressed.In this case, the Red subframe is addressed by a pulse R1 which overlapsthe Blue subframe addressed by a pulse R2, as in the case of FIG. 4 b,and the Green subframe is addressed by a pulse R3. In accordance withthe method, the pastel colours maintain their original luminance level.

Shown in FIG. 5 c is an example of addressing a completely white pixelor one having a 60% or 90% grey level, as shown. In this case, thepulses for the Red, Blue and Green subframes are identical and of thesame duration, the duration varying depending on the desired grey level.

An example of implementation of an electronic circuit allowing themethod described above to be employed will now be described withreference to FIGS. 6, 7 and 8.

As shown more particularly in FIG. 6, which shows a circuit 100 usingthe invention for the colour red, the preceding colour value, namely thevalue R2, is sent to a look-up table, labelled LUT1 101, which outputsan overlap datum proportional to the period of overlap with the Bluesubframe. This datum is sent to the input of a circuit 102 whichsubtracts the overlap value from the current blue colour value B1. AB-overlap value is obtained as output from the circuit 102. This valueis sent as input to a comparator 103, more particularly to the +terminal of the comparator 103, the-terminal of which is connected toearth. The output from the comparator 103 is sent to two switchingcircuits 105, 106, 107 as trigger value for the switches 105, 106 and107. Moreover, one of the inputs of the switch 105 receives the previouscolour value R2, which is also sent to a circuit 104 that fulfils acorrection function, which will be described below. The circuit 104 alsoreceives the B-overlap value.

The output from the correction circuit 104 is sent to the other inputterminal of the switching circuit 105, which gives as output a valueR_(OUT) for the red output value. The previous colour value R2 is alsosent to a second look-up table LUT2 102 which gives, as output, anoffset value labelled Offset. This offset value Offset is sent to oneinput terminal of an adder 108, the other terminal of which receives ablue colour value B₁, so as to give, as output, a B+Offset colour valuewhich is sent to one of the inputs of the switching circuit 106, theother input of which is connected to earth. A blue colour value labelledB₂ is obtained as output from the switching circuit 106.

Moreover, a green colour signal labelled G_(IN) is sent to a circuit 109fulfilling a correction function, which receives the signal B-overlap asinput. The output from the correction circuit 109 is sent to one of theinputs of a switching circuit 107, while the other input of theswitching circuit 107 receives the colour value G_(IN). The switchingcircuit 107 is controlled by the signal coming from the comparator 103and gives a colour value signal G₁ as output.

FIG. 7 shows three circuits 100, 200, 300 identical to the circuit shownin FIG. 6, making it possible to carry out the method described above insuccession for the colours red, F_(R), blue, F_(B), and green, F_(G). Asshown in FIG. 7, the output B₂ and the output G₁ coming from the circuit100 are sent to the circuit 200 and a red colour value R_(IN) is sent asinput to the circuit 200. The circuit 200 makes it possible to obtainthe blue colour value B_(OUT). The same applies in the case of thecircuit 300, which receives as input the green colour value G₂ and thered colour value R₁ output by the circuit 200 and a blue colour valueB_(IN) and which gives as output the green colour value G_(OUT) and thered colour value R₂ and the blue colour value B₁ which are fed back intothe circuit 100 carrying out the improvement function in the case of thered colour R_(OUT).

The operation of the circuits in FIGS. 6 and 7 will be explained below.Thus, the red colour value R₂ is sent to the table LUT1 100 whichincludes reference values depending on the response time of the materialforming the display, the content of this table being explained below.

The overlap value is subtracted from the blue colour value B₁ so as togive B-overlap. If this value is greater than zero, the switchingelement 105 outputs the colour value R₂ onto R_(OUT) and the B+Offsetvalue is added to the blue channel B₂, the switch 106 being positionedas shown in FIG. 6. The green value G₁ as output is also equal to theinput value G_(IN), the switch 107 being positioned as shown in FIG. 6.If the B-overlap value is less than zero, the switch 106 switches to theearthed input and the blue value B₂ is set to zero. In this case, theswitches 105 and 107 switch to their input connected to the correctionfunction circuits 104 and 109, respectively, and the values of theoutputs R_(OUT) and G₁ are reduced by an amount that maintains theoriginal tint value, while reducing the luminance.

As will be explained below, the correction function consists of a blockbased on multipliers that reduce the red and green values, in the caseof FIG. 6, depending on the B-Overlap value.

In the embodiment in FIG. 6, the overlap data and the offset data areobtained from two tables LUT1 101 and LUT2 102. However, these datacould be calculated from one another by solving, for example, the systemof two equations in two unknowns below:S_(overlap)%=f(t_(video))S_(offset)%=g(t_(video))=>S_(offset)%=g(f⁻¹(S_(overlap)%)).

As explained below, the Overlap and Offset values depend on the responsetime of the liquid crystal material and on the duration of the subframe.

An illustration of the values contained in the table LUT1 101 will nowbe given with reference to FIG. 8. FIG. 8 characterizes an example of aliquid crystal LC having linear rise and fall times in order to simplifythe demonstration.

The label S_(offset) corresponds to a lack of luminance in the bluesubframe labelled Blue, induced by the rise-time and fall-timecharacteristics of the liquid crystal. To correct this, it is necessaryto add a time offset to the blue value. This offset is labelledt_(offset). S_(overlap) corresponds to the contamination of the greenvalue with the blue value. Two cases may occur, as described above:

-   -   the pixel colour is not saturated. In this case, the blue colour        is not modified, nor is the green colour;    -   the pixel colour must be saturated. In this case, the blue value        must be reduced by a value corresponding to S_(overlap)=green        value.

Consequently, the other two colour values must be reduced by the samevalue in order to maintain constant tint. This is the role of thecorrection functions in FIG. 6. If S_(overlap) and S_(offset) arecalculated as a function of the video signal of the preceding subframe,T_(video), the rise and fall times, Tr and Tf and the subframe period T,the calculation results in: $\begin{matrix}{S_{overlap} = {\frac{1}{2}{( {t_{video} + T_{f} - T} )^{2} \cdot \frac{L_{\max}}{T_{f}}}}} & {{{{If}\quad t_{video}} + T_{f}} \geq T} \\{{S_{overlap}\%} = {\frac{S_{overlap}}{S_{\max}} = {\frac{1}{2}\frac{( {t_{video} + T_{f} - T} )^{2}}{T \cdot T_{f}}}}} & {{{If}\quad t_{video}} \geq {T - T_{f}}} \\{{S_{overlap}\%} = 0} & {{{If}\quad t_{video}} \leq {T - T_{f}}} \\{{S_{offset}\%} = {\frac{S_{offset}}{S_{\max}} = {\frac{1}{2}\frac{{T_{r}( {T - t_{video}} )}^{2}}{T \cdot T_{f}}}}} & {{{If}\quad t_{video}} \geq {T - T_{f}}} \\{{S_{offset}\%} = {\frac{S_{offset}}{S_{\max}} = {\frac{1}{2}\frac{T_{r}}{T}}}} & {{{If}\quad t_{video}} \leq {T - T_{f}}}\end{matrix}$

S_(overlap) and S_(offset) are loaded into the tables LUT1 101 and LUT2102. If the video signal is encoded over N bits, the percentage valuemust be multiplied by 2^(N)-1.

One way of carrying out the correction function, which may beimplemented in the circuits 104 and 109 of FIG. 6, will now be describedwith reference to FIGS. 9 and 10. The upper part of FIG. 9 shows atheoretical video signal having a first pulse RV of duration equal toone subframe, a second, very short pulse BV during the next subframe anda third pulse GV of duration less than the duration of the thirdsubframe. In this case, as regards luminance and as shown in part B inFIG. 9, there is an overlap value coming from the first subframe, namelythe Red subframe in the embodiment shown, with the second or Bluesubframe. Since the value of the blue colour is very low, an error isobserved which does not allow the tint to be maintained. This is shownby the dotted line T, which crosses the falling edge of the Redluminance pulse. The same applies to the colour green. In this case, acorrection function must be active in order to maintain the tint. Thiscorrection function reduces the value of the preceding colour (namelyred in the embodiment shown) in such a way that the overlap value isequal to the value desired for the colour blue. This is shown in FIG.10, in which it may be seen that the dotted line T crosses the fallingedge when the blue value is approximately equal to zero. This correctionfunction may be used with adders and multipliers, depending on thetransfer below, taking as assumption the fact that the data is encodedover eight bits. When B-Overlap<0: $\begin{matrix}{R_{out} = {R_{2} \times ( {1 - \frac{{Overlap} - B_{1}}{255}} )}} \\{B_{2} = 0} \\{G_{1} = {G_{of} \times ( {1 - \frac{{Overlap} - B_{1}}{255}} )}}\end{matrix}$

The same function can be applied to the other colours.

It is obvious to a person skilled in the art that the above exampleshave been given merely as an illustration.

1. Method of improving the luminous efficiency of a sequential-colourmatrix display, the display being driven using an addressing method ofthe pulse width modulation or PWM type, characterized, for each pixel ofa subframe, by the following steps: comparison of the pixel colour valueof the preceding subframe with a reference value so as to provide anoverlap value depending on the period of overlap with the currentsubframe; if the pixel colour value of the current subframe less theoverlap value gives a positive value, a time offset is to be added tothe pixel colour value of the current subframe; if the pixel colourvalue of the current subframe less the overlap value gives a negativevalue, the pixel colour value of the current subframe is forced to bezero.
 2. Method according to claim 1, wherein, if the pixel colour valueof the current subframe less the overlap value gives a negative value,the pixel colour value of the preceding subframe and the colour value ofthe next subframe are modified so as to maintain the original tint,while at the same time reducing the luminance.
 3. Method according toclaim 1, wherein the above steps apply in succession to each sequentialcolour of a frame.
 4. Method according to claim 1, wherein the pixelcolour value of a subframe depends on the width of the PWM-typeaddressing pulse.
 5. Method according claim 1, wherein the referencevalue depends on the response time of the material forming the display.6. Method according to claim 1, wherein the time offset depends on theresponse time of the material forming the display and on the duration ofthe subframe.
 7. Method according to claim 5, wherein the referencevalue and the time offset are stored separately in two separate tables.8. Method according to claim 5, wherein the reference value and the timeoffset are calculated from each other.